The email from June 2015 was reportedly sent from Zuckerberg to then-Oculus CEO Brendan Iribe, Facebook COO Sheryl Sandberg, and six other Facebook execs. It details plans to buy the game-engine developer Unity, a deal that ultimately never materialised.

Zuckerberg wrote in his 2015 email that Facebook had to be at the forefront of VR/AR to secure a dominant position quickly before its competitors move into the space.

"Our goal is not only to win in VR / AR, but also to accelerate its arrival," he wrote.

Zuckerberg also noted that an acquisition of Unity would have given Facebook a certain amount of leverage against big app platforms.

"If we own Unity, then Android, Windows and iOS will all need us to support them on [sic] larger portions of their ecosystems won't work. While we wouldn't reject them outright, we will have options for how deeply we support them."